1. Field of the Invention
This invention relates to an improved binder formulation for use in conjunction with a tri-aryl pyrazoline compound in the charge transport layer of layered electrophotographic plates.
2. DESCRIPTION OF THE PRIOR ART
It is known in the art of electrophotography that in substantially all modern electrophotographic reproduction processes images are formed upon a photoconductive insulating surface by first subjecting the photoconductive surface to a uniform charge and thereafter selectively exposing the surface to light. Because of the nature of the photoconductive material, that portion of the surface which has been exposed to the light is rendered electrically conductive so as to cause the charge to dissipate in the illuminated areas and to leave behind an electrostatic latent image. The image so obtained is thereafter developed, or rendered visible, by any of a number of different means such as by contact with a developer, including a carrier and an electroscopic material known in the art as a "toner".
Various photographic imaging elements have been described in the patent literature which may be employed for the purpose of carrying out the foregoing phenomenon. In practice, the electrophotographic imaging element may be a homogeneous layer on a support or it may be a multi-layered structure including a layer of charge generating photoconductive material and layers of other materials. A substantial number of electrophotographic imaging elements including multiple layers are illustrated in the patent literature. For example, U.S. Pat. No. 3,041,166 discloses a layered structure consisting of an inorganic vitreous selenium photoconductor overlaid with an insulating photoconductive polymer. U.S. Pat. No. 3,165,405 discloses a structure designed for reflex imaging utilizing a two-layered zinc oxidebinder structure. U.S. Pat. No. 3,394,001 discloses an electrophotographic element including a conductive substrate carrying a photoconductive material being both underlaid and overlaid by an electron donor dye. U.S. Pat. No. 3,537,906 illustrates an electrophotographic element including photoconductive double layers containing an organic, photoconductive insulating layer between the substrate and photoconductive vapor deposited selenium. In U.S. Pat. No. 3,598,582 is described a composite photosensitive device adapted for reflex exposure which employs a layer of organic photoconductive particles arrayed on a supporting substrate and overcoated by a layer of organic charge transport material. More recently many patents have been issued which utilize a composite structure consisting of a conductive substrate, a charge generation layer and an organic charge transport layer as taught by U.S. Pat. No. 3,598,582. These include U.S. Pat. Nos. 3,713,820; 3,725,058; 3,824,099; 3,837,851; 3,839,034; 3,850,630 and 3,898,084.
It can therefore be seen that the patent literature has recognized that a particularly useful electrophotographic imaging element is a layered photoconductor which is composed of layers among which may be designated a charge transport layer, a charge generating layer and a conductive substrate. Thus, for example, in U.S. Pat. No. 3,837,851, where such a photoconductor is disclosed, it is acknowledged that while the theory underlying the discrete layered photoconductor is not known with certainty, it is believed that, for instance, in the situation where the charge transport layer is on top of the charge generating layer and the electrophotographic plate is thereby negatively charged with electrons, and because the charge transport layer has a very low absorbtivity in the light range in which the usual photocopies illuminate the photoconductor, the light which impinges on the surface of the photoconductor is transmitted through the charge transport layer to the charge generating layer. In response to exposure to the light which has been transmitted to it, the charge generating layer in turn is believed to generate responsively both holes and electrons. While the electrons are bled off through the conductive substrate, the holes from the charge generating layer are transported back through the charge transport layer to the surface where they combine with the electrons, the electrons not readily dissipating under dark conditions, so as to neutralize the surface charge. By exposure of the surface of the photoconductor in this manner, a remaining charge pattern is created which may thereafter be rendered visible.
It can therefore be seen that photoconductivity in the context of the layered photoconductor as described above, and as noted in U.S. Pat. No. 3,837,851, involves at least two steps: (1) the generation of a charge and (2) the transporation of a charge. As to the latter, U.S. Pat. No. 3,837,851 teaches that a charge transport layer which has been found to be particularly useful when used in conjunction with a separate charge generating layer is one which is formed from photoconductive pyrazoline compounds manifesting the following formula: ##STR1## wherein n is zero or one, and A, A.sup.1 and A.sup.2 are each aryl radicals. It is further stated therein that it is preferred that n=1 in which case the materials may be classified chemically as styryl pyrazolines and that one or more of the aryl groups be substituted, most preferably with groups known in the art to be electron donating groups. The most preferred substituent groups are methoxy, ethoxy, dimethyl amino, diethyl amino and the like. When using the aforementioned pyrazoline compounds as the charge transport layer, it is known that although they are film forming and thus may be used alone to form the charge transport layer, it is generally preferred to use these compounds in conjunction with a binder material for the purpose of imparting mechanical strength to the charge transport layer.
While many binder materials, including various acrylic resins and polycarbonates, are known to be generally useful in photoconductor applications, for instance, as disclosed in U.S. Pat. Nos. 3,527,602, 3,549,362, 3,684,548 and 3,770,428, and notwithstanding that polycarbonates, polyester resins, polyvinylidene chloride and polystyrene are known to be useful individually as binder materials in layered photoconductors wherein a tri-aryl pyrazoline compound is employed as the charge transport layer, i.e., U.S. Pat. No. 3,837,851, it has been observed that when the binder materials are used in a layered photoconductor comprising a conductive substrate, a charge generating layer and a charge transport layer as taught by the prior art, the resulting photoconductor, though possessing mechanical properties which are better than those where no binder material at all is used, nevertheless exhibits very serious disadvantages and drawbacks which makes such a photoconductor of very limited use in actual practice.
For instance, it has been observed that when only a polycarbonate binder material is employed in a tri-aryl pyrazoline charge transport layer, a film is formed upon the surface of the photoconductor. This film, which may for convenience be designated as "toner film," is believed to be caused by the adherence of the toner material, used to render the latent electrostatic charge visible, to the surface of the photoconductor. The toner film so adhered upon the photoconductive surface is not readily cleaned off and brings about streaking in the photocopies. Such a film also brings about a loss of the required sensitivity as well as a quick decrease in the electrical characteristics of the photoconductor such that the number of useful copies obtainable from such a photoconductor is very limited. Further, and notwithstanding the admonition in the prior art that binder materials should be employed "for mechanical reasons," as stated in U.S. Pat. No. 3,837,851, it has been observed that when only a polycarbonate is employed as the binder material, the surface of the photoconductor is very soft, is easily damaged and soon loses the smoothness necessary to yield good photocopies. In fact, the surface of the charge transport layer soon wears away after the production of only a relative few copies.
Similarly, it has been observed tht when only an acrylic resin binder is employed in a tri-aryl pyrazoline charge transport layer, a clear film is very quickly formed upon the surface of the photoconductor. Such a clear film, which may be analogized to a floor-wax film, is believed to be caused by adhered Teflon which is not easily removable from the surface of the photoconductor, the Teflon being deposited from both the brush used to clean the surface as well as from the toner carrier material. This clear film is as detrimental as the toner film formed by the adherence of the toner material, and, like the toner film, also results in a decrease in sensitivity and the concurrent limitation in the number of useful copies obtainable. It has also been observed that when an acrylic resin is so employed, streaks develop upon the photocopies. It is believed that the streaking is caused by a like streaking upon the surface of the photoconductor surface due to the clear film deposited thereon. Further, the use of an acrylic resin in the charge transport layer has been observed to render the surface very brittle and easily subject to cracking and breakage even under the normal and routine handling that such a photoconductor is subjected to.
Further, the aforementioned filming problems become even more acute as the temperature to which the photoconductive surface is exposed is elevated. It would therefore be desireable to not only be able to overcome the filming problems, but also to be able to deal more successfully with such filming at the elevated temperatures.
Accordingly, there is demonstrated a need for a binder formulation which is compatible for use with tri-aryl pyrazoline compounds in the charge transport layer of a layered photoconductor and which will overcome the aforementioned disadvantages. It is to a particular and improved binder formulation for use in conjunction with a layered photoconductor that the instant invention is directed.